Notes Payable (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended					1 Months Ended	12 Months Ended
	Dec. 31, 2013 Y	Dec. 31, 2013 Letter Of Credit [Member]	Dec. 31, 2013 Short-term Debt [Member]	Dec. 31, 2013 Floating Rate Debt [Member]	Dec. 31, 2012 Floating Rate Debt [Member]	Dec. 31, 2013 5.45% Notes Due 2013 [Member] Senior Unsecured Notes [Member]	Dec. 31, 2013 Line of Credit [Member]
Amount of current borrowing capacity under unsecured credit facility							$ 500
Unsecured Credit Facility, Expiration Date							Sep. 22, 2015
Optional extension period of unsecured credit facility							September 2016
Maximum borrowing capacity under unsecured credit facility		100					750
Maximum term of bid rate loans (days)							180
Lesser of amount stated or the amount available under the line of credit							250
Outstanding letters of credit under credit facility		11.3					0
Available amount under unsecured credit facility							488.7
Other Short-term Borrowings			0
Debt Instrument, Face Amount	250
Debt Instrument, Interest Rate, Stated Percentage	4.25%					5.375%
Discounted notes payable face amount	99.814%
Notes payable, effective interest rate	4.27%					5.45%
Proceeds From Notes Payable Net Of Issuance Costs	247.4
Repayment of unsecured debt using proceeds from debt issuance						$ 200
Weighted average interest rate on floating rate debt	4.40%			1.00%	1.10%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years)	6.9